In recent years there has been a tremendous demand for adding motor propulsion to what had previously been manual propelled machines. One example of such a device is ride-on and walk-behind power equipment. Walk-behind devices were once solely push-propelled. However, recently more models have become self-propelled. Examples of such equipment are lawnmowers, lawn vacuums, snowblowers, trimmers, edgers, concrete and asphalt cutters and the like. A number of factors have driven the move to self-propulsion, such as a desire for larger equipment which requires less effort and allows for more detailed control. Speed of operation is another factor that has driven the increase demand for self-propelled equipment. Current self-propulsion systems for walk-behind equipment generally fall into two categories, simple and complex.
In a simple type drive system, a drive shaft or belt is connected to a single drive axle which drives two wheels, one at each end of the axle. Because the wheels are on a common shaft, they rotate at the same rate. The principal deficiency with this type of device is that no differential rotation is permitted between the drive wheels. As a result, the machine is not efficient during cornering, which requires the outer wheel to travel a greater distance than the inner wheel. Because the outer wheel travels farther than the inner wheel in the same amount of time, the outer wheel should rotate faster than the inner wheel. When the outer and inner wheels are fixed to a common axle, however, that differential rotation is not permitted. The result is that either the inner wheel is driven faster or the outer wheel is driven slower than is optimum for the speed of the vehicle. In either case, cornering the equipment requires one of the wheels to slip or skid. That results in premature wear of the wheel.
Difficulty with cornering and wheel slippage are two major disadvantages with using equipment having drive wheels fixed to a common axle. Additionally, effort by the operator must be provided to overcome the ground-engaging forces to allow one wheel to slip. Furthermore, wheel slippage can cause damage to the surfaces on which the equipment is operating, as well as accelerated tire wear. For instance, turning a lawnmower with this type of drive system damages the turf under the slipping wheel.
Complex drive systems for self-propelled, walk-behind power equipment generally provide a differential between the pair of drive wheels. The differential permits independent or differential rotation of the drive wheels on an axle when the user corners. Many drive systems with differentials use some form of an overrunning clutch to transmit torque when needed to a driven shaft, while allowing a wheel to turn faster than the motor drive when necessary. One successful use of an overrunning clutch in an all terrain vehicle incorporates overrunning clutches where the wheel hub mounts to the axle, thus allowing each wheel to independently disengage when required.
Conventional complex differentials and overrunning clutches are generally costly to manufacture and, thus, relegated to more expensive vehicles, such as cars and four wheel drive vehicles.
A need exists for a less complex and less expensive bi-directional overrunning clutch that can be used in various self-propelled machines and light duty vehicles, such as snowblowers, lawn mowers, golf carts, and concrete and asphalt cutters.
Commonly assigned U.S. Pat. No. 6,722,484 describes a bi-directional overrunning clutch that allows the motor to drive both wheels in either direction when proceeding in a straight line, and on corners allows the motor to drive the slower (inside) wheel, while allowing the outside wheel to turn faster, without the need for a complex differential. U.S. Pat. No. 6,722,484 is incorporated herein by reference in its entirety. The clutch disclosed in U.S. Pat. No. 6,722,484 has proven very satisfactory. However, there is still room for further improvement.